1. Field of the Invention
This invention relates to an apparatus for metallurgically treating steel gears by thermomechanical means to produce high strength and accurate contact surfaces through a net shape finishing process.
2. Description of the Prior Art
Highly loaded precision gears are normally manufactured by carburizing the surface of low carbon steel gears and reaustenitizing the entire gear prior to hardening by rapid quenching to below the temperature at which the diffusionless transformation process that creates the hardened martensitic structure proceeds to completion, the so-called M.sub.f temperature. For medium to high carbon steel gears only the surface of the gears are reaustenitized prior to quenching to produce the hardened martensitic structure. The hardened gears are then finished to net shape by grinding, skiving or other hard finishing operations. A method has been proposed in U.S. Pat. No. 4,373,973 in which a carburized gear is reaustenitized and quenched to above the start of the martensite transformation temperature, the so-called M.sub.s temperature, rolled and then quenched to martensite before any diffusional decomposition can form from the metastable austenite. This invention includes light cold-working or burnishing to complete the transformation of remaining austenite. However, no specific process details are described that produce the required metallurgical state for through-hardened, medium- or high-carbon steel gears. Nor does that disclosure describe a specific apparatus which can accomplish this process.
In reducing the concept of U.S. Pat. No. 4,373,973 to practice, we have discovered that certain critical basic issues must be addressed for a metallurgically sound and dimensionally accurate gear to be produced. To achieve metallurgically sound structures, the surface decarburization and attending oxide network characteristic of gas carburizing must be significantly reduced or eliminated. This is because, unlike conventional gear finishing, the outermost surface layers are not removed during the final finishing operation. Metallurgically sound gears also have retained austenite levels of less than 10 percent. Retained austenite is particularly prevalent with high carbon or high hardenability steel compositions. Highly accurate gear teeth require very precise control of the deformation process to minimize root slivers, lead direction errors, and profile direction errors. The present invention includes apparatus and methods to control both metallurgical quality and dimensional accuracy during thermomechanical gear finishing to produce the quality required of precision gears.
Gear finishing by rolling uses two types of motion: (1) in-feed motion in which the axes of the workpiece and the die are brought together to a fixed position to engage the mesh of each to accomplish the deformation process and (2) through-feed motion in which the axes are translated parallel to each other after meshing or synchronization at a fixed distance of separation. In conventional cold rolling operations either one or other method is used. In-feed motion is used primarily for helical gears in which there is no way to compensate for tooth-to-tooth dimensional variations. Through-feed motion is required for spur gears but conventional gear finishing machines do not compensate for dimensional variations along the lead direction. In order to successfully accomplish thermomechanical finishing by rolling, both processes must be used simultaneously and very accurate coordination between the two motions must be maintained to compensate for tooth-to-tooth and lead variations. As a prerequisite for precise control of the rolling die and workpiece during processing, the initial fixed setting must also be precisely controlled. For instance, axial out-of-plane misalignment between the workpiece and tool can produce lead errors. In-plane misalignment between workpiece and tool can lead to profile errors, a non-uniform profile contour along the lead direction, as well as lead errors.
Therefore, to produce gears by thermomechanical processing which possess desirable metallurgical properties and dimensional accuracy, it is necessary to maintain precise control over the environment, thermal conditions and mechanical actions.